The present invention generally relates to the art of controlling the tack of materials, and more particularly pertains to altering the tack of polymers used as adhesives and pitch.
People throughout our society have become increasingly aware and concerned about the environmental issues that plague the world. The depletion of the ozone layer, the rain forests, and clean water are just a few of the environmental issues that are being addressed. One approach in addressing these issues includes preserving resources by recycling them. Consequently, the recycling industry has become instrumental to serving this need.
In the recycling field, one area of increasing interest is the reuse of wastepaper. Millions of tons of wastepaper are generated every year in the United States. Recycling this wastepaper can save countless trees, as well as provide other ecological and economic benefits. However, the key to reuse of this wastepaper is the removal of contaminants from the wastepaper, thereby facilitating the use of recycled or secondary fibers from the wastepaper.
The paper recycling industry encounters a variety of contaminants in wastepaper. Many of these contaminants adhere to paper fibers and therefore may cause problems during the recycling process. One such contaminant is xe2x80x9cstickiesxe2x80x9d, which were used originally as paper adhesives or tacky adhesives. Stickies typically are classified as hot melts, pressure-sensitive adhesives (PSAs), latexes, and binders. Pitch is another contaminant associated with both virgin and secondary fibers. Pitch is a part of the extractives from wood, and is released during pulping.
Contaminants may cause operational and product quality problems. Specifically, contaminants may be deposited on wires, felts, press rolls, and drying cylinders of paper machines. In addition, contaminants may hinder bonding of fibers, increase web breaks, and reduce product quality in the papermaking process. Consequently, contaminants must be controlled in order to improve papermaking operations and product quality.
Tack is the sticky property of paper adhesives, paperboard adhesives and glue coating materials. The tack of an adhesive and the adhesive""s ability to bond to another surface is dependent, in part, upon the surface energy of the adhesive. Reducing the tack of contaminants can minimize the propensity of the contaminants to attach to paper machine surfaces, thereby leading to fewer operational problems.
Various prior art methods are used to reduce the tack of the contaminants. Some methods use repulpable or recyclable adhesives. More common methods include chemical additives for modification, detackification, or pacification of the contaminants. For instance, detackification of contaminants is frequently accomplished by adding minerals, such as talc, or surface-active chemicals. These minerals and surface-active chemicals attach to the surface of the contaminants and alter their surface properties, thereby causing tack reduction. This method of tack reduction is described in a publication entitled, xe2x80x9cSuccessful Approach in Avoiding Stickies,xe2x80x9d by S. Abraham, Tappi J., 81:2 79-84 (1998), which is incorporated herein by reference. Nonetheless, chemical additives can be very expensive and may cause other problems in the papermaking process, such as a decline in product quality.
Mechanical methods for controlling contaminants include dispersion, screening and cleaning. Dispersion is a technique by which contaminants are broken up into smaller and smaller particles until they are invisible in the final product. Unfortunately, the overall appearance of a product may be diminished greatly by the presence of contaminants. In addition, when the product containing contaminants is wound, sticking may occur between adjacent layers.
Screens and centrifugal cleaners are typically used to remove stickies, pitch and debris from the fiber stream. In general, screens are used to physically separate fiber from contaminants based on the differences between the sizes and shapes of contaminants and the holes or slots in the screen. One problem is that screens cannot remove contaminants that are either smaller than the size of the screen hole or deformable enough to pass through the screen hole. Centrifugal cleaners separate contaminants from fiber primarily on differences between the specific gravities of the fiber and the contaminant. However, separation is poor if the specific gravity of the contaminant is similar to the specific gravity of the fiber.
Therefore, there is a need for a system and method for improving the removal efficiency of contaminants, such as stickies and pitch, from a fiber stream. In addition, there is a need for a system and method that can detackify contaminants by altering, without the use of chemicals, the surface properties of the contaminants. There is yet another need for a system and method that alters the tack of materials inexpensively and simply.
The present invention solves the above-described needs by providing a system and method for altering the tack of a material by the exposing the material to an electrical discharge in a liquid medium.
In one aspect, the present invention provides a method for altering the tack of a material by immersing a material in a liquid medium and introducing an electrical discharge in the liquid medium, wherein the material is exposed to the electrical discharge and the electrical discharge causes a reduction in the tack of the material. The exposure to an electrical discharge or spark can be repeated until the tack of the material is reduced to a desired level of tackiness.
The liquid medium is typically selected from a group consisting of whitewater, water, and a pulp slurry. The material is preferably a polymer used as an adhesive or the material may also be pitch. The polymer is selected from a group consisting of pressure sensitive adhesives, hot melts, latexes, and binders. Stickies or pitch may be suspended alone or attached to a surface of an object, such as a fiber, a metal object, a plastic object, and other machine surfaces. Moreover, the electrical discharge produces energy of about 0.1 to 25 kJ.
In another aspect, a method for reducing the tack of a contaminant in a liquid medium is described. Specifically, a high voltage/high current store of energy is discharged in a liquid medium containing a contaminant, wherein the energy is discharged in a predetermined time period and the energy causes the tack of the contaminant to be reduced.
In addition, multiple high voltage/high current stores of energy can be discharged in the liquid medium containing the contaminant until the tack of the contaminant reaches a desired level. The multiple high voltage/high current stores of energy may discharged from the same energy source or from different energy sources. In the case of multiple stores of energy emanating from different energy sources, the multiple high voltage/high current stores of energy may be discharged either synchronously or asynchronously. The energy is discharged from a sparker.
The predetermined time period preferably ranges from about 5 microseconds to 500 microseconds. The high voltage is preferably in the range of about 500 V to 20,000 V. The high current is preferably in the range of about 10,000 A to 100,000 A.
In another aspect, a system for altering the tack of sticky contaminants in paper machine and pulp recycling operations so as to improve paper product quality in paper-making processes and to reduce operational problems is described comprising: a power supply supplying high current/high voltage electricity to a capacitor bank, where the capacitor bank is connected to the power supply and stores the high current/high voltage electricity supplied by the power supply; a chamber containing a liquid medium with sticky contaminants; and at least one pair of electrodes being submersed in the liquid medium contained in the chamber, where the at least one pair of electrodes is cabled to the capacitor bank and releases in the liquid medium at least one spark of the high current/high voltage electricity stored in the capacitor bank, wherein the sticky contaminants in the liquid medium are exposed to the release of the high current/high voltage electricity from the at least one pair of electrodes, thereby altering the tack of the sticky contaminants.
The release of high current/high voltage electricity produces energy of preferably about 0.1 to 25 kJ. Moreover, the duration of the release of high current/high voltage electricity ranges from preferably about 5 microseconds to 500 microseconds.